I. Field of the Invention
This invention relates to a holographic element of the type comprising a substrate absorbent to light of a recording wavelength, and a holographic material which is sensitive to light of a recording wavelength, readable at a reading wavelength and usable to form a volume hologram, the substrate and the holographic material forming a composite substrate structure.
II. Prior Art and Other Considerations
A volume hologram may be formed in the holographic material by exposing the holographic material in a known way to light at the recording wavelength. The holographic element may then be read by exposing the holographic material to light at the reading wavelength.
Holographic elements of this general type are disclosed in more detail in British patent application number GB8917455.1 in the name of the present Applicant to which reference is directed. In holographic elements of this known type, the holographic material is deposited directly onto the substrate. After the holographic material has been exposed to light at the recording wavelength and subsequently developed, a holographic device is formed by laminating the substrate and holographic material to a mirrored cover glass. The mirrored cover glass is fixed to the holographic material and the substrate by means of an adhesive. Typically an epoxy adhesive is used for index matching and humidity protection.
A disadvantage of this known holographic element is that the substrate, generally coloured glass, must be of a very high optical quality while being transmissive at the reading wavelength and absorbent at the recording wavelength. In addition, the mirrored surface of the mirrored cover glass is susceptible to damage during processing.